1. Field of the Invention
This invention relates to sodium-sulphur cells and is concerned more particularly with the construction of the cathode current collector.
2. Prior Art
Sodium-sulphur cells have a solid electrolyte of beta-alumina separating a liquid sodium metal anode from a liquid cathodic reactant which includes sulphur. This cathodic reactant has a composition which depends on the state of charge or discharge of the cell. As the cell discharges, sodium ions pass through the electrolyte into the cathodic reactant to combine with negatively charged sulphide ions to form sodium polysulphides; the cathodic reactant however is commonly referred to as the sulphur electrode. It is necessary to inject and extract electrons from the sulphur electrode and this is done by means of a porous conductive body, such as a graphite or carbon felt matrix. The porous conductor acts as a large-area electrode surface at which electrons can be supplied to or removed from the sulphur or polysulphides during the cell reaction. As the electrical conductivity of such a porous material is low, a current collector has to be electrically connected to this matrix to enable an external circuit to be connected to the cathode of the cell.
The present invention is concerned more particularly with this cathode current collector. Under the electrochemical conditions prevailing in the sulphur electrode, even stainless steel is subject to corrosion. Heretofore however stainless steel has been considered to be the best material to employ. The corrosion of the steel has several deleterious effects. For example, sulphur which would otherwise be available for reaction in the cell is consumed in the formation of corrosion products. If the cell capacity is controlled by the sulphur, then such consumption of sulphur reduces the cell capacity.